1. Technical Field of the Invention
The present invention relates to medical magnetic resonance imaging (MRI), particularly, imaging to provide images of objects in motion, such as blood, using multi-echo sequences. Specifically, the present invention relates to imaging that uses an FSE (Fast Spin Echo) method or a FASE (Fast Asymmetric SE) method developed therefrom in order to not only image flows of such objects as blood so as to be depicted more steadily but also to depict a distribution of flow velocities with the help of a technique similar to a phase contrast method.
2. Description of Prior Art
Magnetic resonance imaging is based on a technique that magnetically excites nuclear spins of an object placed in a static magnetic field with an RF signal at a Larmor frequency thereof, acquires an MR signal emanated due to the excitation, and reconstructs an image on the basis of the MR signal.
In recent years, as one tomographic imaging technique that is frequently used in the field of MRI, an FSE method is known. The FSE method has a feature of being able to valid influences of the non-uniformity of a static field. Especially, a recent tendency is that echo train spacing (ETS) can be shortened thanks to the development of hardware techniques. Therefore, pulse sequences based on the FSE method that are shorter in echo train spacing or FSE-system pulse sequences developed therefrom are used to have depicted, without contrast mediums, targets in motion, such as blood, that were only tentatively imaged in the past. For example, papers concerning such study are shown by xe2x80x9cM. Miyazaki et al., A novel MR angiography technique: SPEED acquisition using half Fourier RARE, JMRI 8: 505-507, 1998,xe2x80x9d xe2x80x9cD W Kaandorp, et al., Three-dimensional Flow Independent Angiography of Aortic Aneurysms using standard Fast Spin Echo, In xe2x80x9cproceedings, ISMRM, 67th Annual Meetingxe2x80x9d Sydney, Australia, p792, 1998,xe2x80x9d xe2x80x9cM. Miyazaki et al., Fresh Blood Imaging at 0.5-T: Natural Blood Contrast 3D MRA within Single Breathhold, In xe2x80x9cProceedings, ISMRM, 6th Annual Meetingxe2x80x9d Sydney, Australia, p780, 1998,xe2x80x9d and xe2x80x9cY. Kassai et al., 3D Half-Fourier RARE with MTC for Cardiac Imaging, In xe2x80x9cProceedings, ISMRM, 6th Annual Meetingxe2x80x9d Sydney, Australia, p806, 1998.xe2x80x9d
These imaging techniques adopt an ECG gating method by which a delay time from the R-wave is appropriately determined at a cardiac temporal phase representing slower velocities of blood flows passing vessels to be imaged, or a phase-encoding direction, which provides a higher depiction, is adjusted to agree with a conventional blood flow direction of interest.
As another approach for conventional blood flow imaging, a nulling technique for gradient moments has been known to suppress consequences of flows thereof.
However, the foregoing imaging methods are not enough for stable blood flow detection. For example, although the depiction is higher in blood flows along the phase encoding direction, it is reported that blood flows in the readout direction are unable to be depicted. Also reported are artifacts, referred to as xe2x80x9cN/2 artifacts,xe2x80x9d due to the oscillation of signals between the even and odd echoes belonging to multi-echoes. Furthermore, because excessive times for switching gradients are required, the nulling method of gradients is practiced only in cases where flows pass at slower velocities, resulting in a limited versatility.
The present invention has been performed in consideration of the drawbacks faced by the foregoing conventional MR imaging. One object of the present invention is to use pulse sequences for multi-echoes including FSE-system pulse trains so as to steadily depict objects in motion, such as blood flow, with no contrast medium, providing images of higher reliability in clinics.
Another object is to make it possible to image objects whose flow velocities range is wide regardless of magnitudes of flow velocities.
In order to realize the above objects, the present invention employs an imaging principle and a construction based on principles, which are as follows. In the following explanations, according to necessities, an exciting RF pulse is merely referred to as a xe2x80x9cflip pulsexe2x80x9d and a refocusing RF pulse as a xe2x80x9cflop pulse.xe2x80x9d
First, the imaging principle will be explained by comparison with conventional techniques.
In the FSE method, when an interval between flip and flop pulses is xcfx84 (i.e., echo train spacing: ETS), it is essential that the first flip-flop interval xcfx84xe2x80x2 be precisely set to be xcfx84xe2x80x2=t/2 and an amount of a gradient pulse applied between flip and flop pulses be precisely half an amount A of gradient pulses to be applied thereafter. Therefore, in cases where a pulse sequence is designed based on the conventional FSE method, as a pulse train to be applied after the flip pulse, the fundamental pulse train of which xcfx84xe2x80x2 and Axe2x80x2 are sufficiently adjusted to xcfx84xe2x80x2=xcfx84/2 and Axe2x80x2=A/2 is repeated.
One example of the FSE-based pulse sequence thus-designed is shown in FIG. 1(b) with its phase diagram shown in FIG. 1(a). This pulse sequence represents only a flip pulse, a plurality of flop pulses and a readout gradient Gr, while slice and phase-encode gradients Gs and Ge are omitted from the drawing.
In the phase diagram shown in FIG. 1(a), its longitudinal axis shows degrees of dephase of magnetic spins and its transverse axis shows time t. Additionally, solid lines extending in oblique directions show the states of transverse magnetization (transverse paths) in which the dephases advance. The transverse dotted lines show the states of longitudinal magnetization (longitudinal paths) which preserve dephase states as the longitudinal magnetization. For the FSE-based pulse sequence that satisfies the foregoing temporal condition xcfx84xe2x80x2=xcfx84/2 and area condition Axe2x80x2=A2, the states of the transverse and longitudinal magnetization are expressed in a neat condition as shown in FIG. 1(a), resulting in MR images with no artifacts.
Conventionally, for imaging an object in motion using the FSE method, an imaging technique for practicing a flow compensation (FC) method or gradient moment nulling (GMN) method with the foregoing temporal and area conditions satisfied is proposed by xe2x80x9cRS. Hinks et al., Gradient Moment Nulling in Fast Spin Echo, MRM 32: 698-706, 1994.xe2x80x9d
However, in this FSE method cooperatively using the FC and GMN methods, additional gradient pulses are required for the ordinal FSE-basis pulses, resulting in, for example, a prolonged interval between the flip and flop pulses (i.e., the echo train spacing: ETS). Namely, as the time necessary for practicing the pulse sequence per excitation, data acquisition efficiency is degraded, prolonging the entire imaging time. Due to this drawback, it will be difficult to steadily catch an object to be depicted, even when the object, such as blood, flows faster.
Moreover, as described above, the echo train spacing ETS is shortened to, for example, 5 [msec] in order to try to obtain images of blood flow or the heart in motion. In this pulse sequence of which ETS is shortened, there is no room for cooperatively using the FC and GMN methods, because the gradient pulses and RF pulses are tightly arranged along the time axis. If this pulse sequence of which ETS is shortened is used, a higher depiction ability of blood flows along the phase encoding direction is obtained, as described before. However, the depiction ability along the readout direction is poor and xe2x80x9cN/2 artifactsxe2x80x9d occur because of the oscillation of signals on the even and odd echoes.
The behaviors of signals, which include xe2x80x9cN/2 artifacts,xe2x80x9d obtained on a conventional FSE-basis pulse sequence is now explained with reference to FIG. 2. This example of the signal behaviors, which was simulated with an FSE-basis pulse sequence whose ETS=5 [msec], flop angle FA=150 degrees, and imaging region=35 [cm], show a flow velocity dependency when assuming that the flow velocity of an object is 30 [cm/s]. In this example, as reflecting the consequences of xe2x80x9cphase shift effects resulting from motionxe2x80x9d due to the readout gradients, the phases and intensities of echo signals are calculated on a phase diagram.
Each data at the upper row in FIG. 2 shows in complex numbers the phases and intensities of loci of signals of the first to thirty-second echoes. These echo signals are Fourier-transformed into real-space reconstructed images, of which data are shown at the lower row in the figure.
From the graphs at the upper row, in the stage of echo signals, there can be seen that 1): as a whole, the phase is shifted relative to the real-part direction as a reference (in cases where an object is at rest, its echo signal is originally oriented in the real-part direction), 2): the signal caused between even echoes and odd echoes oscillates, and 3): the echo signal is shifted in the complex coordinate.
In the real-space data which have undergone Fourier transformation and which are each shown at the lower row in the figure, responsively to the foregoing phenomena 1) to 3), there occur that 1xe2x80x2): the phase is shifted similarly to the echo signal, 2xe2x80x2): in addition to an original image signal, an xe2x80x9cN/2xe2x80x9d artifact is caused, and 3xe2x80x2): an image signal diminishes (at a flow velocity of v=50 [cm/s], an image disappears).
The upper and lower rows in FIG. 3 show examples of signal behaviors, respectively, in the k-space and real space, which renders the dependency on flop angles and are simulated in the same manner as above (the flow velocity is supposed to be v=15 [cm/s]) as the flop angles are changed. As shown from those graphs, when the flop angle FA is lowered, the signal oscillation caused between the even and odd echoes become more complicated, resulting in that an image is caused to disappear in states of FA=60 degrees or less.
Under such signal behaviors, one large cause that brings about troubles on images, which has been reported so far, results form the signal oscillation described in the above article 2). Thus, FIGS. 4(a)-4(f) explain this signal oscillation, in which an attention is paid to spin echoes, i.e., echo components generated when dephased spins invert in response to the application of an RF pulse.
FIGS. 4(a)-(f) exemplify the behavior of spin echoes according to a pulse sequence in a conventional FSE method. When supposing that the initial phase shift of the first echo (a phase shift produced from the application of a flip pulse to the generation of the first echo) be xc3x8xe2x80x2 and a phase shift produced from the k-th echo to the xe2x80x9ck+1xe2x80x9d echo as for kxe2x89xa71 be Ø, the conventional sequence provides Øxe2x80x2≈0.8Ø, whose behaviors are shown in FIGS. 4(a)-(f).
From the stage of FIG. 4(a) where the phase shift has been accumulated up to an initial phase shift xc3x8xe2x80x2, the phase shift is inverted to xe2x88x92xc3x8xe2x80x2 responsively to the application of the first flop pulse (FIG. 4(b)). Until the application of the second flop pulse, the phase is shifted by an amount of xc3x8 to a phase of xe2x80x9cxc3x8-xc3x8xe2x80x2xe2x80x9d (FIG. 4(c)). The application of the second flop pulse causes the phase to invert to xe2x80x9cxc3x85xe2x80x2-xc3x8xe2x80x9d (FIG. 4(d)). Again, the phase becomes xc3x8xe2x80x2 by phase shifting by an amount of xc3x8 during an interval to the application of the third flop pulse (FIG. 4(e)). Hereafter, the similar behaviors are repeated. Therefore, it can be understood that, as shown in FIG. 4(f), the even echoes and odd echoes are oscillated over xc3x8/2 residing therebetween.
In the above explanation, a premise was xc3x8xe2x80x2≈0.8xc3x8, which is able to be derived from a conventional FSE method shown in FIG. 1, where, when ETS=5[ms] is designated, xcfx84xe2x80x2≈1.2+2.0=3.2[ms] and xcfx84≈2.0+2.0=4.0[ms] are established, so that the ratio of those two figures is 3.2:4.0=0.8:1. As for the FSE method, since the ratio of areas of gradient pulses is maintained at a certain relationship, the phase shift is proportional to an amount of xe2x80x9cinterval between application of gradients x velocity.xe2x80x9d In this invention, xcfx84xe2x80x2 is a temporal difference between one instant at the gravity of a readout gradient waveform applied between the flip and the flop, and another instant at which the gravity of a gradient waveform for producing the first echo is achieved in its portion from the start of its application to the center of an echo. Additionally, xcfx84 is a temporal difference between one instant at which the gravity of a readout gradient waveform for producing the k-th echo is achieved in its portion from the center of an echo to the end of its application, and another instant at which the gravity of a gradient waveform for producing the xe2x80x9ck+1xe2x80x9d-th echo is achieved in its portion from the start of its application to the center of another echo.
The basic principle of imaging of the present invention is to make the foregoing initial phase shift Øxe2x80x2 agree or substantially agree with Ø/2 (the xe2x80x9cagreementxe2x80x9d used herein means a degree of agreement under which, at least, even a bit of advantages of the present invention can be obtained). The behaviors of echoes when this condition of the phase shift, Øxe2x80x2=(or ≈) Ø/2 (hereinafter, referred to as a xe2x80x9cØ/2 conditionxe2x80x9d) is met are illustrated in FIGS. 5(a)-(f) with the notation equivalent to the foreoging FIGS. 4(a)-(f). Namely, from the stage of FIG. 5(a) where the phase shift has been accumulated up to an initial phase shift Øxe2x80x3=Ø/2, the phase shift is inverted to xe2x88x92Øxe2x80x2=xe2x88x92Ø/2 responsively to the application of the first flop pulse (FIG. 5(b)). Until the application of the second flop pulse, the phase is shifted by an amount of Ø to a phase of xe2x80x9cØ/2xe2x80x9d (FIG. 5(c)). The application of the second flop pulse causes the phase to invert to xe2x80x9cxe2x88x92Ø/2xe2x80x9d (FIG. 5(d)). Again, the phase becomes xe2x80x9cØ/2xe2x80x9d by phase shifting by an amount of Ø during an interval to the application of the third flop pulse (FIG. 5(e)). Hereafter, the similar behaviors are repeated. Therefore, it can be understood that, as shown in FIG. 5(f), differently from the conventional method shown in FIG. 4, the echoes become stable at a phase of xe2x80x9cØ/2.xe2x80x9d
In the foregoing pulse sequence on the conventional FSE method, there existed a restriction in sequence design, as described before, and easy variations are therefore unavailable to the sequence.
Hence, the present invention proposes an imaging technique that uses a pulse sequence capable of overcoming such a drawback.
According to a practical configuration of the present invention, as a first invention, there is provided an MR imaging method comprising the steps of: performing a pulse sequence including an excitation pulse exciting spins of an object to be imaged, a plurality of pulses generating a plurality of echoes by inverting the excited spins a plurality of times, and a gradient pulse set such that a gradient moment to be accumulated before a generation time to be referenced of a given echo selected from the plurality of echoes is substantially half a gradient moment accumulated between echoes generated after the generation time; acquiring the plurality of echoes generated in response to the performance of the pulse sequence; and producing an MR image by using at least part of echoes included in the acquired plurality of echoes and generated after the given echo.
Preferably, the performing step is a step performed with the gradient pulse set such that, among the plurality of echoes, an amount Mxe2x80x2 of a j-th (j=1, 1, 2, . . . ) gradient moment accumulated up to an n-th (n=1, 2, 3, . . . ) echo is approximately half an amount M (considered to be approximately constant when kxe2x89xa7n) of a j-th gradient moment from a k-th (k=n, n+1, n+2, . . . ) echo to xe2x80x9ck+1xe2x80x9d-th echo.
For example, the j-th gradient moment is a velocity moment corresponding to j=1. Further, for example, the MR image is produced using all echoes generated after the n-th echo in the plurality of echoes.
Furthermore, for instance, the pulse sequence is a pulse sequence that causes multi-echoes to be generated in response to application of a plurality of refocusing pulses. In this case, by way of example, the j-th gradient moment is a velocity moment corresponding to j=1.
A further example is that, as the multi-echo type of pulse sequence that uses the refocusing pulses, an pulse sequence on a fast SE method that allows all indirect echoes to be included and superposed. For example, the pulse sequence is a single shot type of pulse sequence.
One preferable mode is that the n-th echo of the pulse sequence is an echo for nxe2x89xa72 and the gradient pulse applied until the xe2x80x9cnxe2x88x921xe2x80x9d-th echo in order to set the gradient moment is changed about either one of time and amplitude thereof. In this construction, for example, the gradient pulse is a readout gradient pulse. Still, it is preferred the readout gradient pulse applied at the first echo to the xe2x80x9cnxe2x88x921xe2x80x9d-th echo of the pulse sequence on the fast SE method is a gradient pulse applied shifted in time, where the gradient pulse is set to be shifted in a time axis direction such that an amount of the shifted time becomes a desired value with signs given the shifted time in accordance with the number of inversions of a refocusing pulse.
Additionally, the pulse sequence may be a pulse sequence on the basis of a DIET (Dual Interval Echo Train) method. In such a case, the pulse sequence on the DIET method can be a pulse sequence in which gradient moment nulling is performed before and after a refocusing pulse to be applied in a first echo train spacing to be anticipated and the condition for setting the gradient moment is satisfied immediately before a first echo.
Still, the pulse sequence may be a sequence set in an order on a centric order technique where an order of phase encoding applied to the echoes is oriented from a central part in a phase encode direction of a k-space to a peripheral part thereof in the k-space. Further, the pulse sequence may be a pulse sequence on the basis of a partial-Fourier method. Moreover, the pulse sequence can be performed using either one of an electrocardiograph gating technique and a peripheral gating technique. In this case, either one of the electrocardiograph gating technique and the peripheral gating technique is set to perform a delay time control in a manner that the pulse sequence is executed at a temporal phase in which a flow velocity in a blood vessel employed as the object is relatively smaller.
As a second invention, provided is an MR imaging method for obtaining an MR image of an object that flows, by using a multi-echo type of pulse sequence, comprising the steps of: producing the pulse sequence by setting a gradient pulse such that, among the plurality of echoes, an amount Mxe2x80x2 of a j-th (j=1, 1, 2, . . . ) gradient moment accumulated up to an n-th (n=1, 2, 3, . . . ) echo is approximately half an amount M (considered to be approximately constant when kxe2x89xa7n ) of the j-th gradient moment from a k-th (k=n, n+1, n+2, . . . ) echo to a xe2x80x9ck+1xe2x80x9d-th echo; and performing the pulse sequence.
Further, as a third invention, provided is an MRI system comprising: means for performing a pulse sequence including an excitation pulse exciting spins of an object to be imaged, a plurality of pulses generating a plurality of echoes by inverting the excited spins a plurality of times, and a gradient pulse set such that a gradient moment to be accumulated before a generation time to be referenced of a given echo selected from the plurality of echoes is substantially half a gradient moment accumulated between echoes generated after the generation time; means for acquiring the plurality of echoes generated in response to the performance of the pulse sequence; and means for producing an MR image by using at least part of echoes included in the acquired plurality of echoes and generated after the given echo. Preferably, the pulse sequence is a pulse sequence in which, among the plurality of echoes, an amount Mxe2x80x2 of a j-th (j=1, 1, 2, . . . ) gradient moment accumulated up to an n-th (n=1, 2, 3, . . . ) echo is approximately half an amount M (considered to be approximately constant when kxe2x89xa7n) of a j-th gradient moment from a k-th (k=n, n+1, n+2, . . . ) echo to a xe2x80x9ck+1xe2x80x9d-th echo.
As a fourth invention, there is provided a computer-readable recording medium in which a multi-echo type of pulse sequence for MR imaging is recorded in the form of a program, wherein the pulse sequence is a pulse sequence in which, among the plurality of echoes, an amount Mxe2x80x2 of a j-th (j=1, 1, 2, . . . ) gradient moment accumulated up to an n-th (n=1, 2, 3, . . . ) echo is approximately half an amount M (considered to be approximately constant when kxe2x89xa7n) of a j-th gradient moment from a k-th (k=n, n+1, n+2, . . . ) echo to a xe2x80x9ck+1xe2x80x9d-th echo.
Further, as a fifth embodiment, provided is an MR imaging method for obtaining an MR image of an object that flows, by using a multi-echo type of pulse sequence including a plurality of refocusing RF pulses, comprising the steps of: performing, under m-piece (m is plural) gradient moments M (M1, M2, . . . , Mm) to be applied, the pulse sequence set such that, among the plurality of echoes, an amount Mxe2x80x2 of a j-th (j=1, 1, 2, . . . ) gradient moment accumulated up to an n-th (n=1, 2, 3, . . . ) echo is approximately half an amount M (considered to be approximately constant when kxe2x89xa7n ) of the j-th gradient moment from a k-th (k=n, n+1, n+2, . . . ) echo to a xe2x80x9ck+1xe2x80x9d-th echo; acquiring the multi-echoes emanated in response to the performance; and imaging j-th information of the object from a plurality of sets of MR data in association with the acquired multi-echoes. As an example, the m-piece gradient moments have an amount representing a difference from a desired gradient moment M0 associated with a phase contrast method. For this case, by way of example, the desired gradient moment M0 is set such that M0=0 to when performing gradient moment nulling.
Additionally, as a sixth invention, provided is an MRI system for obtaining an MR image of an object that flows, by using a multi-echo type of pulse sequence including a plurality of refocusing RF pulses, comprising: means for performing, under m-piece (m is plural) gradient moments M (M1, M2, . . . , Mm) to be applied, the pulse sequence set such that, among the plurality of echoes, an amount Mxe2x80x2 of a j-th (j=1, 1, 2, . . . ) gradient moment accumulated up to an n-th (n=1, 2, 3, . . . ) echo is approximately half an amount M (considered to be approximately constant when kxe2x89xa7n) of the j-th gradient moment from a k-th (k=n, n+1, n+2, . . . ) echo to a xe2x80x9ck+1xe2x80x9d-th echo; means for acquiring the multi-echoes emanated in response to the performance; and means for imaging j-th information of the object from a plurality of sets of MR data in association with the acquired multi-echoes.
One example of advantages which are obtained through the performance of a pulse sequence on the FSE method satisfying the xe2x80x9cxc3x8/2xe2x80x9d condition realized by the above configuration is shown in FIG. 6(b) as the behaviors of echo signals (supposing that an object flows at a velocity of v=30 [cm/s] and an flop angle FA is 150 degrees). In the figure, the upper graph shows the behaviors of echoes in the complex coordinates, while the lower one shows the behaviors of echo signals Fourior-transformed into the real space. For comparison, in a similar way, FIG. 6(a) shows the behaviors of echo signals obtained through the performance of a pulse sequence on the conventional FSE method. As to the phenomena about the problems resulting from the conventional method, it is understood that the oscillations of signals between the even echoes and the odd echoes is largely decreased (refer to the upper graphs in the FIGS. (a) and (b)), leading to a large decrease in the xe2x80x9cN/2xe2x80x9d artifacts due to the oscillations (refer to the lower graphs in the FIGS. (a) and (b)).
In the foregoing explanation, the behaviors of spins have been expressed by the use of the term xe2x80x9cphase shift.xe2x80x9d By the way, the phase sift is proportional to a product of a gradient moment and flow velocity. If a pulse sequence is given, the gradient moment is constant, with the result that the phase shift is proportional to a flow velocity. For example, if xc3x8=30 degrees at a flow velocity of 10 [cm/s], the phase shift can be exemplified as follows.
The xe2x80x9cxc3x8/2xe2x80x9d condition is satisfied whatever the flow velocity is. From a physical viewpoint, the term xe2x80x9cgradient momentxe2x80x9d is correct and should be used instead of the xe2x80x9cphase shiftxe2x80x9d if a strict expression is required, but the term xe2x80x9cphase shiftxe2x80x9d is still effective and can be used as a generous expression. Therefore, the term xe2x80x9cphase shift,xe2x80x9d which is a more concrete concept, is used in the following description.
In the present invention, an imaging technique that meets xe2x80x9cxc3x8/2xe2x80x9d condition is referred to as a VIPS (Velocity Independent Phase-shift Stabilization) method.
By the way, in this application, as a seventh invention, there is provided an MR imaging method for obtaining an MR image of an object by using a multi-echo type of pulse sequence including a plurality of refocusing RF pulses, wherein a flop angle that the plurality of refocusing pulses provide a magnetization spin is set to be a smaller value of 90 degrees or less. For instance, as the multi-echo type of pulse sequence using the refocusing pulses, a pulse sequence on a fast SE method by which indirect echoes are superposed on the echoes is used.